1. Field of the Invention
The present invention relates to an ion concentration measuring device for measuring a cation, particularly a sodium ion or a potassium ion, in a biological sample.
2. Description of the Related Art
Methods for analyzing the concentration of an electrolyte (such as a sodium ion or a potassium ion) in a biological sample, particularly blood serum or blood plasma, include flame photometry and ion selective electrode method. Of these, the ion selective electrode method is now widely used since it can determine ion concentration in a sample only by immersing the ion selective electrode in a sample solution in conjunction with a reference electrode. In the medical field, a device for the ion selective electrode method is built in a biochemical automatic analyzer, taking the advantage that such device can be miniaturized and automated.
A liquid-membrane ion selective electrode has an ion-selective membrane to which an ionophore (or ligand) is added. Each ligand is capable of selectively interacting with a certain ion, and various kinds of ions can be measured with different ligands added to the ion-selective membrane according to the target ions. An ion-sensitive material called a “neutral carrier” is used for a cation selective electrode for the sodium ion, the potassium ion, and the like. Crown ether, which is a typical neutral carrier, selectively recognizes a cation fitted to a hole size of a crown ring. Generally, 12-crown-4 is suitable for a lithium ion; 15-crown-5, for the sodium ion; and 18-crown-6, for the potassium ion. Moreover, crown ether can have better cation selectivity than monocrown ether by sandwiching a cation between two crown rings. In this case, 14-crown-4 is suitable for the sodium ion; and 15-crown-5, for the potassium ion.
Because of the problem of interference made by a lipophilic anion, the ion selective electrode using a crown ether compound additionally contains a plasticizer or an anion excluder mixed with the ligand in polyvinyl chloride (PVC), that acts as a membrane support material. Typically, dioctyl phenyl phosphonate (DOPP) or nitrophenyl octyl ester (NPOE) is used as the plasticizer, and sodium tetraphenylborate that is lipophilic salt is used as the anion excluder. In addition, the ligand has a complex structure with a hydrophobic side chain (for example, a long alkyl chain) added thereto, in order to prevent elution of the ligand from within the lipophilic PVC membrane from deteriorating performance of the ion selective electrode. Since the ion-selective membrane in contact with an internal standard solution is immersed in the sample solution in actual measurement, fabrication of the ion selective electrode requires an operation of attaching an ion-selective membrane to an internal standard solution receptacle, besides an operation of forming the ion-selective membrane.
There has been proposed the ion selective electrode in which the ligand is immobilized on the surface of a gold electrode, in order to solve the above problem with the liquid-membrane ion selective electrode; to simplify the structure and a fabrication method of the ion selective electrode; and to miniaturize an analytical instrument in the future. As an example of a sodium ion selective electrode, it has been reported that a mercapto crown ether compound has been immobilized on the gold electrode as neutral carrier type ligand (See J. Am. Chem. Soc., 120 (1998): 4652-4657). Moreover, as an example of a lithium ion selective electrode, it has also been reported that the neutral carrier type ligand has likewise been immobilized on the surface of the gold electrode, utilizing a bond between thiol and gold (See Anal. Chem., 78 (2006): 7132-7137, and WO2006/113440.)